Two interrelated subjects are under investigation: 1) characterization of the types, properties and functions of endogenous retroviruses in mice and the mechanisms of their regulation, expression and, in the case of the recombinant (MCF) viruses, their generation, and 2) determination of the elements and processes involved in development of murine lymphoma. These projects, utilizing biological, genetic, serological, and molecular approaches, have helped to unravel some of the details of leukemogenesis in AKR mice and have partially defined the biology of MCF viruses. Recent work has revealed that MCF viruses can have restricted tropisms for lymphocyte subpopulations in vivo and that there are close associations between cellular differentiation and MCF biology. A dominant antigen of certain MCF viruses was found to be conformationally determined using monoclonal antibodies. This was the first report of a major antigen of murine retroviruses being conformational in nature. With others, the existence of MCF-like env gene sequences in chromosomal DNA of mice has been demonstrated. Some of these sequences have been further studied in newly generated panels of MCF viruses. It was found that distinct endogenous sequences recombine specifically with different ecotropic viruses in generating MCF's.